The invention relates to a wind turbine having a non-FBG, optic fibre sensor system and method for detecting deformation of a wind turbine rotor blade, and particularly to a temperature compensated system and method.
Wind turbine components are subject to deformation or strain from a number of sources, such as the accumulation of particulates like dirt or ice, their own weight, and the force exerted by the wind itself. Consequently, it is important that the strain on components be monitored to ensure that they remain fit to operate over their intended working lives. Wind turbine rotor blades are particularly susceptible to such phenomena.
It is known to detect the strain on wind turbine components using Fibre Bragg Grating (FBG) sensors, such as those described in patent applications GB 4640953, GB 4640954 and GB 4640955. An FBG sensor is an optical fibre in which an optical grating is formed. The grating itself is typically a periodic variation in the refractive index of the fibre, tuned to reflect a particular wavelength of light.
The part of the optical fibre having the grating is attached to the region of the wind turbine component where the strain is to be measured. It is attached in such a way that any deformation or strain experienced by the component is transmitted to the fibre and to the grating. Deformation and strain causes the spacing of the grating to change, and causes a detectable change in the wavelength of light reflected back by the grating. Various arrangements are known for inserting light into the FBG sensors and for extracting and analysing the output.
FBG sensors pose a number of disadvantages. Forming the grating in the fibre is costly, typically requiring the removal of the optic fibre coating, and subsequent re-coating. FBG sensors also are typically site specific, and only sense deformation at the location of the grating.
The paper entitled “Fatigue strength of glass reinforced polyester (GRP) laminates with embedded optical fibres” by Alfredo Guemes and Jose M Menendez, published at the Third ICIM/ECSSM '96 at Lyon, ISBN 0-8194-2165-0/96 discloses an alternative technique for determining the deformation of a wind turbine component, based on interferometric techniques. Incident light is input to two optical fibres and subsequently recombined to give an interference pattern based on a Michelson Interferometer arrangement. As one of the optical fibres is subject to strain, the resulting interference will depend on the strain.
We have appreciated that there is a need for a more robust and cost effective solution for detecting deformation of a wind turbine component, that is able to address both large and small deformations with accuracy.